1. Field of the Invention
The present invention relates to copolymers which are suitable as materials for optical devices such as plastic lenses and prisms and for substrates or base plates of optical information recording media such as video disks, compact disks, and computer information storage disks. More particularly, the invention relates to copolymers which have high transparency, high heat resistance, low hygroscopicity, good moldability, and low double refractivity, i.e., low indexes of birefringence. These copolymers are used chiefly through injection molding or extrusion.
2. Description of Related Art
Transparent resins are used as materials for optical purposes. In recent years, Fresnel lenses, aspherical lenses, optical disks, etc., made of transparent resins have drawn attention and hence various copolymers have been studied as materials for the manufacture of the optical devices cited above.
Resins which are commercially available and are used today as materials for optical purposes, include polycarbonate and polymethyl methacrylate resins. Polymethyl methacrylate resins, although used because of their high transparency, excellent weather resistance, and low double refractivity, have drawbacks in that the hygroscopicity of these resins is high and upon moisture absorption, they are likely to undergo dimensional changes or deformations such as warps, and in that the heat resistance of these resins is not very high. Hence high temperatures, considerably lower the mechanical strength of these resins and cause their deformations, such as warps.
Polycarbonate resins, although used in view of their low hygroscopicity and high heat resistance, have the drawbacks of poor moldability and high double refractivity. Therefore both types of resins are largely restricted in their applications.
Japanese Patent Application Kokai Nos. Sho 57-33446, Sho. 57-162135, and Sho. 57-108012 propose copolymerization of methyl methacrylate with aromatic vinyl monomers including styrene for the purpose of reducing the high hygroscopicity of polymethyl methacrylate resins. However, when the hygroscopicity is sufficiently reduced by the copolymerization with an aromatic vinyl monomer such as styrene, a further increase in the content of the aromatic vinyl monomer would result in a highly birefringent copolymer, which cannot be used as a material for optical purposes. In addition, such copolymers have lower heat resistance, and hence would be unsuitable in applications in which it is necessary for the material to have similar or higher heat resistance than that of polymethyl methacrylate resins.
Japanese Patent Application Kokai Nos. Sho. 57-186241, Sho. 58-127754, Sho. 58-154751, Sho. 59-1518, and Sho. 60-10410 propose copolymerization of methyl methacrylate with cyclohexyl methacrylate for the purpose of reducing the high hygroscopicity of polymethyl methacrylate resins without increasing their double refractivity. However, when the hygroscopicity is sufficiently reduced by the copolymerization with cyclohexyl methacrylate, a further increase in the content of the cyclohexyl methacrylate would markedly lower the heat resistance and heat stability of the copolymer and render it to be a very brittle copolymer. Hence this copolymerization is problematic in terms of practical applications.
Japanese Patent Application Kokai Nos. Sho. 63-17915 and Hei. 1-178811 (corresponding to U.S. Ser. No. 07/246,823 filed on July 26, 1988 now abandoned) propose copolymers of methyl methacrylate with 4-t-butylcyclohexyl methacrylate, such as 4-t-butylcyclohexyl methacrylate, in an attempt to correct the drawbacks of the methyl methacrylate-cyclohexyl methacrylate copolymer. These copolymers containing t-butylcyclohexyl methacrylate, although being improved in heat resistance over the copolymer containing cyclohexyl methacrylate, have only corrected the drawbacks to a limited extent.
Japanese Patent Application Kokai Nos. Sho. 59-227909 and Sho. 60-115605 propose copolymerization of methyl methacrylate with isobornyl methacrylate or fenchyl methacrylate for the purpose of reducing the hygroscopicity and improving the heat resistance of the respective copolymers formed. However, when isobornyl methacrylate or fenchyl methacrylate is copolymerized with methyl methacrylate, the heat resistance of the copolymer formed is not improved to any great extent and in addition, the mechanical strength as well as the heat stability thereof is also greatly lowered. This would lead to the problem during the melting and molding of the copolymer, in that it would tend to decompose and foam or undergo notable coloration.
On the other hand, U.S. Pat. No. 4,742,123 proposes a thermoplastic, non-cross-linked copolymer having six-membered ring acid anhydride units represented by the formula ##STR1## and a Vicat softening temperature of 50.degree. to 175.degree. C. and also being characterized with good heat stability and a copolymer of said six-membered ring acid anhydride with cyclohexyl methacrylate.
Copolymers containing such a six-membered ring acid anhydride units are improved in heat resistance when the content of the anhydride units is high. However the hygroscopicity of these copolymers becomes higher as the anhydride unit content is increased. Hence these copolymers are not adaptable for applications where it is necessary for the material to have high heat resistance and low hygroscopicity.
Further, Japanese Patent Application Kokai No. Sho. 61-49325 (which was filed by the present applicant) proposes a copolymer comprising units of methyl methacrylate, aromatic vinyl, methacrylic acid, and six-membered ring acid anhydride. Japanese Patent Application Kokai No. Sho. 63-264613 discloses that this copolymer can be used for substrates in optical devices such as disks. Further, although this copolymer is characterized with satisfactory heat resistance and low hygroscopicity properties, it cannot be used in cases wherein the copolymer used would also be required to have a low double refractivity.
As stated above, presently, no resin has been found that satisfies performance requirements on the whole.